1. Field of the Invention
The present invention relates, in general, to a transmission device for farm tractors, having a plurality of gear trains in a transmission case and designed for utilizing the engine power as travelling power at a travelling mode or PTO (power Take Off) power at a PTO mode having a small volume and simple construction suitable for the small-sized agricultural tractor.
2. Description of the Prior Art
In typical farm tractors of several tens of horsepower(hp), an engine power leading unit and main and sub-change gear units are coupled to each other from the front to the back of the transmission case, so they selectively utilize the engine power as travelling power at a travelling mode or PTO power at a PTO mode. The main change gear unit has gear trains capable of transmitting the engine power to the wheels of a tractor for forward and reverse travelling. Meanwhile, the sub-change gear unit has both a travelling power output shaft and PTO power transmitting gear trains, thus selectively driving the implements of the tractor when the tractor performs PTO operations. The transmission device, designed for selectively transmitting the travelling power or the PTO power of several tens of horsepower, has to be provided with appropriate units capable of performing the travelling or PTO operation.
For example, a typical transmission device, used in farm tractors of Tongyang TA series of Korea, comprises a plurality of cooperating units: an engine power leading unit, a main change gear unit, a super low-speed change gear unit, a sub-change gear unit, a forward/reverse drive unit, an independent PTO clutch unit, and a front wheel drive unit.
The above Korean transmission device is designed for transmitting the engine power to a hollow drive shaft through a specifically designed and splined clutch, thus allowing the engine power to be transmitted to PTO drive shaft from a main clutch cover part. In the above transmission device, the PTO power is controlled by an independent PTO clutch, while the travelling power is controlled by a main clutch.
The main change gear unit is designed for transmitting the engine power while changing the rotational speed of the power between several gear shifting stages by bringing a plurality of gears of different sizes into selective engagement with each other in accordance with selected speed changing steps.
In the above transmission device, the sub-change gear unit has a dual or triple shaft structure with one shaft commonly performing a super low-speed changing action and a sub-speed changing action. The transmission device also has a mid-PTO power transmitting unit which is set in a transmission case.
The forward/reverse drive unit comprises five gears in addition to a drive gear engaging with a counter gear. The counter gear also engages with a forward drive gear, while a reverse drive gear, mounted on the same shaft as that of the forward drive gear, engages with a reverse drive idle gear. The reverse drive idle gear comes into engagement with the counter gear, thus allowing the forward/reverse drive unit to select a forward or reverse movement of the tractor.
In the independent PTO clutch unit used for selectively transmitting the power of the engine to a PTO drive shaft, a plurality of power transmitting shafts and gears are independently positioned and operated. In the case of a four wheel drive tractor, the engine power is transmitted to the front wheels of the tractor as follows. That is, the engine power is transmitted from a gear, specifically used for driving the front wheels, to a sub-change gear shaft.
Thereafter, the engine power is distributed from the sub-change gear shaft or another shaft so as to drive the four wheels of the tractor at the same time.
However, such a typical transmission device for farm tractors is problematic in that the specifically designed and expensive clutch increases the production cost of the transmission device. The above clutch also lengthens the transmission device, thus causing operational vibrations on the PTO drive shaft and enlarging the volume of the transmission device. Therefore, the transmission device with such a clutch is not suitable for being used with a small-sized tractor. In addition, the main change gear unit has to be provided with complex gear trains, consisting of a plurality of gears having different sizes, so the main change gear unit regrettably has a complex construction and increases the production cost of the transmission device, and deteriorates productivity of the transmission device. Such complex gear trains of the main change gear unit can also confuse a user while repairing the transmission device, thus being inconvenient to the user.
The dual or triple shaft structure of the sub-change gear unit prevents the power transmission device from being small sized or simplified. In addition, the super low-speed change gear unit is integrated with the sub-change gear unit, so it is impossible to selectively operate the transmission device without allowing the engine power to be processed by the super low-speed change gear unit.
The forward/reverse drive unit comprises five gears in addition to the drive gear, thus having a complex construction and increasing the production cost of the transmission device.
In addition, the independent PTO clutch comprises a plurality of power transmitting shafts and gears which are independently positioned and operated. In the above transmission device, the front wheel drive unit is designed for driving the four wheels of a tractor at the same time by transmitting the engine power from the front wheel drive gear to the sub-change gear shaft and distributing the engine power from the sub-change gear shaft or another shaft. Therefore, the front wheel drive unit complicates the construction of the power transmission device and increases the production cost of the transmission device.